An organic electroluminescent device in the simplest structure is generally constituted of a light-emitting layer and a pair of counter electrodes sandwiching the said light-emitting layer. The device functions by utilizing the following phenomenon; upon application of an electrical field between the electrodes, electrons are injected from the cathode and holes are injected from the anode and they recombine in the light-emitting layer with emission of light.
In recent years, organic thin films have been used in the development of organic EL devices. In particular, in order to enhance the luminous efficiency, the kind of electrodes has been optimized for the purpose of improving the efficiency of injecting carriers from the electrodes and a device has been developed in which a hole-transporting layer composed of an aromatic diamine and a light-emitting layer composed of 8-hydroxyquinoline aluminum complex (hereinafter referred to as Alq3) are disposed in thin film between the electrodes. This device has brought about a marked improvement in the luminous efficiency over the conventional devices utilizing single crystals of anthracene and the like and thereafter the developmental works of organic EL devices have been directed toward commercial applications to high-performance flat panels featuring self-luminescence and high-speed response.
In another effort to enhance the luminous efficiency of the device, the use of phosphorescence in place of fluorescence is investigated. The aforementioned device comprising a hole-transporting layer composed of an aromatic amine and a light-emitting layer composed of Alq3 and many other devices utilize fluorescence. The use of phosphorescence, that is, emission of light from the excited triplet state, is expected to enhance the luminous efficiency three to four times that of the conventional devices utilizing fluorescence (emission of light from the excited singlet state). To achieve this objective, the use of coumarin derivatives and benzophenone derivatives in the light-emitting layer was investigated, but these derivatives merely produced luminance at an extremely low level. Europium complexes were also investigated in trials to utilize the excited triplet state, but they failed to emit light at high efficiency. In recent years, as is mentioned in patent document 1, a large number of researches are conducted with the objective of enhancing the luminous efficiency and extending the lifetime while giving priority to utilization of organic metal complexes such as iridium complexes.    Patent document 1: JP2003-515897 A    Patent document 2: JP2001-313178 A    Patent document 3: JP2002-352957 A    Patent document 4: JP H11-162650A    Patent document 5: JP H11-176578A    Patent document 6: WO2007/063796    Patent document 7: JP2007-129206 A    Patent document 8: WO2007/137725
In order to enhance the luminous efficiency by utilizing phosphorescence, a host material to be used together with a dopant material becomes important. Of the host materials proposed thus far, a typical example is 4,4′-bis(9-carbazolyl)biphenyl (hereinafter referred to as CBP), a carbazole compound cited in patent document 2. When CBP is used as a host material for tris(2-phenylpyridine)iridium complex (hereinafter referred to as Ir(ppy)3), a phosphorescent material emitting green light, the balance of electrical charges in the light-emitting layer is destroyed and excess holes flow out to the side of the cathode on account of the electron transport property being inferior to the hole transport property in the case of CBP and the result is lowering of the luminous efficiency due to lowering of the recombination probability in the light-emitting layer. Furthermore, in this case, the recombination zone in the light-emitting layer is limited to a narrow space in the vicinity of the interface on the cathode side. Consequently, in the case where an electron-transporting material, such as Alq3, whose lowest triplet energy level is lower than that of Ir(ppy)3 is used, there may arise a possibility that the luminous efficiency becomes lower due to transfer of the triplet energy from the dopant to the electron-transporting material.
On the other hand, 3-phenyl-4-(1′-naphthyl)-5-phenyl-1,2,4-triazole (hereinafter referred to as TAZ), disclosed in patent document 3, is also proposed as a host material for a phosphorescent organic EL device. As the hole transport property is inferior to the electron transport property in the case of TAZ, the light-emitting zone is on the side of the hole-transporting layer. In this case, the chosen hole-transporting material influences the luminous efficiency of Ir(ppy)3. For example, the use of 4,4′-bis[N-(1-naphthyl)-N-phenylamino]biphenyl (hereinafter referred to as NPB), a material in widespread use for its good performance, high reliability, and long lifetime, in the hole-transporting layer causes a problem that the triplet energy is transferred from Ir(ppy)3 to NPB reflecting the relationship of the lowest triplet energy level between the two and the luminous efficiency becomes lower.
Furthermore, compounds like CBP and TAZ readily undergo crystallization and agglomeration with the resultant deterioration of the shape of thin film. In addition, the Tg of such compounds is difficult to merely observe because of their high crystallinity. The instability of the shape of thin film in the light-emitting layer exerts an adverse influence on the device such as shortening of the lifetime and lowering of the heat resistance.
As the aforementioned examples indicate, it can readily be understood that a demand is created for host materials that possess simultaneously a high hole transport property and a high electron transport property and are well balanced in the electrical charges (hole and electron) transport properties. Furthermore, it is desirable that the host materials are endowed with electrochemical stability, high heat resistance, and good stability in the amorphous state.
Further, although patent documents 4, 5, and 6 disclose the use of a certain kind of indolocarbazole compounds in organic EL devices and patent documents 7 and 8 likewise disclose the use of a certain kind of phosphorus oxides in organic EL devices, there is a demand for compounds of better properties that are suitable for use in organic EL devices.